Electric Current

Charges on the move!

What makes a charge move?

• Electric current moves due to potential difference

• Electric potential difference is measured as voltage– Potential difference is measured in volts (V)

• The flow of charges due to potential difference is current electricity

What makes charges move?

• Charges must have a pathway to move• Pathway is called a circuit• Circuit must have certain parts:

– Source (area of high potential)– Path (usually wire)– Load (something that does work using the moving

charges)– Sink (area of low potential)

What makes charges move?

• Source of potential difference may be direct current

• Batteries produce direct current due to a chemical reaction

• Voltage produced by chemical reaction can travel through wire to load

• Two kinds of battery:– Dry cell: two elements and chemical paste– Wet cell: two metals and liquid electrolyte

What makes charges move?

• Source of potential difference may be generator

• Generator moves wire in magnetic field to create alternating current (AC)

How many charges move?

• Amount of moving charges is called current• Current measured in Amperes (amps) (A)• Current is number of charges moving past in

one second (Coulombs/sec)

Why do charges lose potential?

• Resistance: similar to friction, resistance slows down charges

• Resistance measured in Ohms (Ω)

• Size of wire affects resistance– Longer wire has more resistance– Thicker wire has less resistance

• Load in the circuit is a resistance

Ohm’s Law

• Relates current, potential difference, and resistance

• Current = voltage / resistance• I = V/R• We can use our circle with this!

Ohm’s Law Example

• Calculate the voltage difference across a 25 Ω resistor if a 0.3 A current is flowing through it?

• Step 1: What do we know?– Know R = 25 Ω I = 0.3 A V = ?

Step 2: What equation will we use?

V = I R

Ohm’s Law Example Continued

• Step 3: Plug in info!– V = (0.3 A)(25 Ω)

• Step 4: Do the math– V = 7.5 V

• Step 5: Check for units!